1. Field of the Invention
This invention relates to vibration sensors and, more particularly, to sensors for detecting failure in rotating machinery.
2. Description of the Prior Art
Bearing materials, although properly manufactured, installed and maintained are, nevertheless, subject to fatigue. Fatigue is the result of shear stresses cyclically applied immediately below the load carrying surfaces and is observed as spalling away of surface metal. In addition to fatigue, premature spalling can be caused by inadequate lubrication, material defects and mechanical damages caused by vibration or electric currents. Therefore, damaged bearings can take the form of cracks, pits, dents, or ridges on either the rollers or inner or outer races.
When a defect comes in contact with a hard surface, such as when a roller defect hits a race or a race defect is hit by a roller, an acoustic impulse is created in the bearing. FIG. 1 illustrates the frequency response of a defective bearing. FIG. 2 illustrates at a, b and c typical defect points indicated by peaks a, b and c of FIG. 1.
Early detection of peaks a, b and c allows the operator of a device containing a bearing to schedule a maintenance shutdown rather than waiting for a bearing failure. Sensing changes of acoustic noise of the shaft speed, indicated in FIG. 1 by peak d also allows early detection of rotor imbalances.
Present technology uses a broad band (flat frequency response) sensor, usually piezoelectric, to detect changes in bearing acoustic noise. An accelermeter is mounted near the bearing and a frequency spectrum is obtained. This procedure requires a recording method, such as an XY recorder or a spectrum analyzer and a skilled interpreter. Because of the expense of such recording instruments, and the large number of bearings in most industrial processes, it is necessary to rotate the instrumentation from one bearing to another, making constant monitoring impossible.
Vibration sensors using magnetostrictive transducers employ crystalline metal alloys which require special annealing procedures and which even then have low magnetomechanical coupling (MMC) factors (in the order of 0.25). Thus, more transducer material is needed to produce a given output signal. Such transducer materials are usually soft mechanically. They are easily scratched and deformed, and crystalline metal magnetostrictive transducer materials having higher MMC factors are expensive and brittle. As a result, undesirable limitations are required to handle these materials during manufacturing operations. In addition, a high magnetic bias field in the order of 300 oersteds is needed for proper operation of present devices. This requires the use of more expensive and powerful magnets to impart the higher levels of magnetization needed for these materials, and if electromagnets are used to provide this magnetic bias, those electromagnets will require more electrical power and can cause heat dissipation problems. Furthermore, crystalline metal alloys employed as magnetostrictive transducers are negative magnetostriction alloys and require substantial amounts of material to support the compressive loads to which they are subjected during operation. Mechanical structures needed to provide those compressive loads are not easily changed to make the structures mechanically resonant with the different vibrational frequencies that are characteristic of different failures. Since each failure can have a different characteristic vibrational frequency many different vibration detectors must be specifically manufactured and inventoried to ensure that a matched detector, tuned to resonate at the characteristic frequency, is available for each particular bearing type. For these reasons, vibration detectors of the type described above have complex and expensive structures and provide output signals too low in magnitude.